Scalable semiconductor waveguide amplifier

ABSTRACT

One embodiment of the present method and apparatus encompasses an apparatus that may have: a predetermined length, the self-imaging semiconductor waveguide having first and second opposed sides; quantum wells disposed within the self-imaging semiconductor waveguide along the length of the self-imaging semiconductor waveguide, the quantum wells being formed of a quantum well gain material; microchannel cooler that extends substantially the width of the self-imaging semiconductor waveguide, the microchannel cooler located adjacent the first side of the self-imaging semiconductor waveguide; and a plurality of pump arrays arranged along the microchannel cooler opposed from the first side of the self-imaging semiconductor waveguide; wherein the quantum well gain material is photopumped through the microchannel cooler.

TECHNICAL FIELD

The invention relates generally to waveguide amplifiers and, moreparticularly, to a scalable semiconductor waveguide amplifier.

BACKGROUND

Optical fiber technology is a core technology for long-distancecommunication systems. Recently, it has been also applied inshort-distance communication systems such as interconnection betweensystems, or short-distance networks. This progress is achieved via theimplementation of a vertical-cavity surface emitting laser (VCSEL).Compared with an edge-emitting laser, the size of the vertical-cavitysurface emitting laser is reduced with less power consumption. Thegreatest benefit of VCSEL is its low fabrication cost. VCSEL can betested on wafers so that any failed devices can be found beforepackaging to save the production cost.

Erbium doped optical amplifiers have been used for many years in opticaltelecommunications networks to boost the strength of an optical signalas it is transmitted along a fiber. While there are many different typesof these amplifiers, they all typically rely on relatively expensivenarrow-band lasers as the pumping sources. It has been recently shownthat in some specially prepared erbium doped optical waveguides, such assilica layers containing Si nanocrystals or nanoclusters dispersed inthe erbium doped glass matrix, that the erbium can be efficientlyexcited using light sources which emit over a broad wavelength range inthe near-infrared or visible portion of the electromagnetic spectrum.Light emitting diodes (LEDs) have been used as a low cost pumping sourcefor these types of amplifiers as there is no longer a strict requirementon the pumping wavelength as is the case in standard erbium doped glass.Such an arrangement has been discussed in a paper by Jinku Lee and JungH. Shun entitled “Optical gain at 1.5 microns in nanocrystal Sisensitized erbium doped silica waveguide using top-pumping 470 nm LEDs”presented at the 2004 OFC meeting. Here an array of LEDs was used topump a section of erbium doped silica waveguide, which had beensensitized with Si nanocrystals.

However, as in the case above, the much lower radiance of the LED,compared to a laser, places a limitation on the efficiency in which thepump power can be transferred to the active medium of the waveguide andconsequently on the output power of the amplifier. Thus for a givenlength of fiber in a communications network, more LED based amplifierswould be required thereby negating any cost savings over laser basedamplifiers.

In general, the prior art taught self-imaging waveguide amplifiers,photopumped VCSELS and Semiconductor Coupled Optical Waveguide Lasers(SCOWL). However, none of the prior art provide, for a optically-pumped,semiconductor waveguide amplifier, high power with diffraction limitedbeams and very high efficiency over a wide range of wavelength.

SUMMARY

One embodiment of the present method and apparatus encompasses anapparatus. The apparatus may comprise: a photopumped self-imagingmultimode waveguide amplifier using a semiconductor gain media; and aplurality of pump arrays arranged along the self-imaging multimodewaveguide.

Another embodiment of the present method and apparatus encompasses anapparatus. This apparatus may comprise: a self-imaging semiconductorwaveguide having a predetermined length, the self-imaging semiconductorwaveguide having first and second opposed sides; quantum wells disposedwithin the self-imaging semiconductor waveguide along the length of theself-imaging semiconductor waveguide, the quantum wells being formed ofa quantum well gain material; microchannel cooler that extendssubstantially the width of the self-imaging semiconductor waveguide, themicrochannel cooler located adjacent the first side of the self-imagingsemiconductor waveguide; and a plurality of optical pump arrays arrangedalong the second side opposed from the first side of the self-imagingsemiconductor waveguide; wherein the quantum well gain material isphotopumped by the pump arrays.

A further embodiment of the present method and apparatus encompasses anapparatus. This apparatus may comprise: a self-imaging semiconductorwaveguide having a predetermined length, the self-imaging semiconductorwaveguide having first and second opposed sides; quantum wells disposedwithin the self-imaging semiconductor waveguide along the length of theself-imaging semiconductor waveguide; first and second heat sinks thatextend substantially the width of the self-imaging semiconductorwaveguide, the first heat sink located adjacent the first side of theself-imaging semiconductor waveguide and the second heat sink locatedadjacent the second side of the self-imaging semiconductor waveguide;and a first plurality of pump arrays arranged along the first heat sinkopposed from the first side of the self-imaging semiconductor waveguide,and a second plurality of pump arrays arranged along the second heatsink opposed from the second side of the self-imaging semiconductorwaveguide.

DESCRIPTION OF THE DRAWINGS

The features of the embodiments of the present method and apparatus areset forth with particularity in the appended claims. These embodimentsmay best be understood by reference to the following description takenin conjunction with the accompanying drawings, in the several figures ofwhich like reference numerals identify like elements, and in which:

FIG. 1 depicts an embodiment according to the present method andapparatus of a photopumped semiconductor waveguide amplifierarchitecture.

FIGS. 2 and 3 show top and end views of the FIG. 1 embodiment.

FIG. 4 depicts a semiconductor waveguide fabrication process for thealternative embodiment of FIG. 1 according to the present method andapparatus.

FIG. 5 depicts one example of a semiconductor waveguide amplifier modelfor the alternative embodiment of FIG. 1.

FIG. 6 depicts an alternative embodiment according to the present methodand apparatus of a photopumped semiconductor waveguide amplifierarchitecture.

FIGS. 7 and 8 show top and end views of the FIG. 1 embodiment.

FIG. 9 depicts a semiconductor waveguide fabrication process accordingto the present method and apparatus.

FIG. 10 is a graph depicting lattice constant versus band gap energy.

FIG. 11 is a three dimensional representation of the thermal resistivityfor In_(1-x), Ga_(x), As_(y), P_(1-y) quarternary alloy over the entirerange of compositions.

FIG. 12 depicts a MIT-LL SCOWL structure.

FIG. 13 an example of one embodiment of the principal of operation.

FIG. 14 depicts one example of a semiconductor waveguide amplifiermodel.

DETAILED DESCRIPTION

Embodiments of the present method and apparatus achieve high power withdiffraction limited beams and very high efficiency over a wide range ofwavelengths. These embodiments provide systems that combine thewavelength flexibility of semiconductor lasers with the high power andefficiency heretofore only available with solid state lasers at fixedwavelengths.

Embodiments of the present method and apparatus provide a scalablesemiconductor waveguide amplifier that is a photopumped self-imagingmultimode waveguide amplifier using a semiconductor gain media. It maybe fabricated by standard epitaxial crystal growth, which is simplerthan VCSEL growth. In these embodiments the photopumping of embeddedquantum wells avoids electrical injection. There is a very low quantumdefect with resonant pumping, and low optical loss for undopedsemiconductor waveguide material. Wavelength may be set by compositionand thickness of quantum wells. Depending on waveguide loss, optical tooptical efficiency may easily exceed 80%.

The use of high thermal conductivity materials enables robust powerscaling and self-imaging assures diffraction limited output beam.

Face-pumping occurs directly or through transparent thermally conductingheat sinks such as SiC or diamond, (thermal conductivity of 3.5 and 22W/cm-K respectively) which enables extremely efficient pump couplingwith good thermal management. The wide band gap semiconductor waveguidematerial has high thermal conductivity (e.g., InP with 0.68 W/cm-K).Gain is adjusted by number of quantum wells and pumping level.

Although the depicted embodiments of the present method and apparatusare primarily continuous wave (CW), embodiments may also be pumped bypulsed lasers (e.g., Nd:YAG, Er:YAG, fiber lasers). A diode pumpedmodule according to the present embodiments may be capable of roughly5-10 kW.

FIG. 1 depicts an embodiment according to the present method andapparatus of a photopumped semiconductor waveguide amplifierarchitecture. This embodiment may be a semiconductor waveguide amplifiersoldered to a microchannel cooler and pumped from the opposite side witha plurality of diode arrays (one sided pumping). Thus, the depictedembodiment may have a self-imaging semiconductor waveguide 102 having apredetermined length, the self-imaging semiconductor waveguide 102having first and second opposed sides (faces) 104, 106. Quantum wells108 may be disposed within the self-imaging semiconductor waveguide 102along the length of the self-imaging semiconductor waveguide 102. Amicrochannel cooler 110 extends substantially the width of theself-imaging semiconductor waveguide 102, the microchannel cooler 110located adjacent the first side 104 of the self-imaging semiconductorwaveguide 102. The microchannel cooler 110 may be fabricated usinghighly conductive alloy materials such as Copper-Molybdenum,Copper-Tungsten or Copper diamond that are compositionally tuned suchthat their thermal expansion matches the semiconductor waveguide 102. Aplurality 114 of pump arrays may be arranged along the microchannelcooler 110 opposed from the first side 104 of the self-imagingsemiconductor waveguide 102.

FIGS. 2 and 3 show top and end views of the FIG. 1 embodiment. FIG. 2more clearly shows the microchannel cooler 110 extending substantiallythe width of the self-imaging semiconductor waveguide 102. Also, shownis the plurality 114 of pump arrays that may be arranged along themicrochannel cooler 110. FIG. 3 also shows coupling optics 103 that maybe located between the pump arrays 114 and the self-imagingsemiconductor waveguide 102.

FIG. 4 depicts a semiconductor waveguide fabrication process accordingto the present method and apparatus. Fabrication and processing may beperformed entirely with an InP or other semiconductor wafer 401.Epitaxial layers 402 may be grown on 3 or 4 inch substrates 403.Free-standing semiconductor waveguide 404 may have quantum wells 405 andmay use quaternary alloys to control band gap and lattice constant. Alsodepicted is the waveguide 406, AR coating 207 and microchannel cooler408.

FIG. 6 depicts an alternative embodiment according to the present methodand apparatus of a photopumped semiconductor waveguide amplifierarchitecture. The depicted embodiment may have a self-imagingsemiconductor waveguide 602 having a predetermined length, theself-imaging semiconductor waveguide 602 having first and second opposedsides (faces) 604, 606. Quantum wells 608 may be disposed within theself-imaging semiconductor waveguide 602 along the length of theself-imaging semiconductor waveguide 602. First and second heat sinks610, 612 extend substantially the width of the self-imagingsemiconductor waveguide 602, the first heat sink 610 located adjacentthe first side 604 of the self-imaging semiconductor waveguide 602 andthe second heat sink 612 located adjacent the second side 606 of theself-imaging semiconductor waveguide 602. A first plurality 614 of pumparrays may be arranged along the first heat sink 610 opposed from thefirst side 604 of the self-imaging semiconductor waveguide 602, and asecond plurality 616 of pump arrays may be arranged along the secondheat sink 612 opposed from the second side 606 of the self-imagingsemiconductor waveguide 602.

FIGS. 7 and 8 show top and end views of the FIG. 6 embodiment. FIG. 7more clearly shows one of the SiC heat sinks 610, 616 extendingsubstantially the width of the self-imaging semiconductor waveguide 602.Also, shown is one of the pluralities 614, 616 of pump arrays that maybe arranged along respectively one of the heat sinks 610, 612. FIG. 8also shows coupling optics 603, 605 that may be located between therespective pump arrays 614, 616 and respective heat sinks 610, 612. Thesemiconductor self-imaging multimode waveguide slab amplifier providesrevolutionary solid state laser capability. The high thermalconductivity media (e.g., InP) permits high power handling, the lowquantum defect assures low thermal load, and the quantum well activemedium is photopumped through transparent heat sinks.

FIG. 9 depicts a semiconductor waveguide fabrication process accordingto the present method and apparatus. Fabrication and processing may beperformed entirely with an InP or other semiconductor wafer 901.Epitaxial layers 902 may be grown on 3 or 4 inch substrates 903.Free-standing semiconductor waveguide 904 may have quantum wells 905 andmay use quaternary alloys to control band gap and lattice constant. Alsodepicted is the waveguide 906, AR coating 907 and wafer bonded to SiCheat sinks 908.

The transparent heat sinks may be attached to the waveguide by using anumber of low absorption optical cements. For example, the opticalcements or bonding agents may be Norland 61, 65 or 71, with Norland 65being the most compliant for thermal cycling. FIG. 10 is a graphdepicting lattice constant versus band gap energy. Quantum wells mayhave both band gap and lattice constant mutually controlled. QuaternaryIII-V alloys (e.g. GaInAsSb, etc.) may be used. The quantum wellthickness also modifies wavelength. FIG. 11 is a three dimensionalrepresentation of the thermal resistivity for In_(1-x), Ga_(x), As_(y),P_(1-y) quaternary alloy over the entire range of compositions. Thus,the quantum well amplifier may function with high heat dissipation.

FIG. 12 depicts a MASS. INSTITUTE OF TECHNOLOGY LINCOLN LABS SCOWLstructure. The present method resembles a high order multimodeimplementation of SCOWL without electrical pumping. Photopumping enableslarge waveguide dimensions and may eliminate free carrier loss.Self-imaging design may result in diffraction limited performance withlarge multimode waveguide.

FIG. 13 shows an example of one embodiment of the principal ofoperation. For an amplifier of thickness d with N quantum wells ofthickness t_(w), an overlap factor T may be calculated as T=Nt_(w)/d.The net gain (G=exp(gTL) depends on the gain coefficient g for each welland the length L of the amplifier. Quantum well gain may depend oncarrier density in a complex way, but can be large (e.g., 100-1000cm⁻¹). Carrier density may also depend on pump intensity and amplifierpower in a complex way. FIG. 14 depicts one example of a semiconductorwaveguide amplifier model. Features may be a large area semiconductorwaveguide, high index contrast, and high order multimode. Waveguidemodes (ignoring quantum wells) may be:

${\phi_{ij} = {\sqrt{\frac{4Z}{ab}}{{Sin}\left( \frac{{\mathbb{i}\pi}\; x}{a} \right)}{{Sin}\left( \frac{j\;\pi\; y}{b} \right)}}},{0 < x < {a\mspace{14mu}{and}\mspace{14mu} 0} < y < b}$i, j = 1, 2, 3, …  n  Eigen  function  normalized  to  unit  power

Signal along length L of amplifier may be calculated as function ofpumping and input field distribution to determine gain and efficiency.The heat flow within the waveguide may be determined to obtain totalheat load. Heat generated in quantum wells and waveguide may bedissipated by the heat sinks. Amplifier may be designed for Talbotself-imaging length (i.e., L=4nd²/λ).

Therefore, a scalable semiconductor photopumped waveguide amplifier mayachieve high power and efficiency. VCSEL, SCOWL and waveguide Nd:YAGhave demonstrated basic principles. The thermal operating limits may be5-10 kW. Self-imaging assures diffraction limited output. Embodimentshave high coupling efficiency with side pumping. O-O efficiency dependson waveguide loss, and may be 80-90%. Pump spectrum is not that criticaland thus, multispectrum pump sources, such as flash lamps, might beused. The wavelength of operation may be adjusted by design and materialcomposition. Lattice matching to binary semiconductors assures goodthermal properties, and cryocooling may be required for operation atMid-IR or Long-IR wavelengths.

The present method and apparatus are not limited to the particulardetails of the depicted embodiments and other modifications andapplications are contemplated. Certain other changes may be made in theabove-described embodiments without departing from the true spirit andscope of the present method and apparatus herein involved. It isintended, therefore, that the subject matter in the above depictionshall be interpreted as illustrative and not in a limiting sense.

1. An apparatus, comprising: a photopumped self-imaging multimodewaveguide amplifier using a semiconductor gain media and having embeddedquantum wells provided along a length of the self-imaging multimodewaveguide amplifier; and a plurality of optical pump arrays arrangedalong arranged along a face of the self-imaging multimode waveguide, forcontinually photopumping the quantum wells through the self-imagingmultimode waveguide to achieve a pumped apparatus with an average powervalue in the range of five to ten kW, for continually photopumping thequantum wells through at least one substantially transparent heat sink,the heat sink disposed between the pump arrays and the waveguide,thereby avoiding electrical injection.
 2. The apparatus according toclaim 1, wherein the apparatus further comprises a microchannel coolerthat extends substantially the width of the self-imaging semiconductorwaveguide, wherein the pump arrays are arranged along the opposed sideof the waveguide from the microchannel cooler.
 3. The apparatusaccording to claim 1, wherein the apparatus further comprises first andsecond heat sinks that extend substantially the width of theself-imaging semiconductor waveguide, and a first plurality of pumparrays arranged along the first heat sink and a second plurality of pumparrays arranged along the second heat sink.
 4. An apparatus, comprising:a self-imaging semiconductor waveguide, the waveguide selected from thegroup consisting of clad and non-clad waveguides, having a predeterminedlength, the self-imaging semiconductor waveguide having first and secondopposed sides; quantum wells disposed within the self-imagingsemiconductor waveguide along the length of the self-imagingsemiconductor waveguide, the quantum wells being formed of a quantumwell gain material; microchannel cooler that extends substantially thewidth of the self-imaging semiconductor waveguide, the microchannelcooler located adjacent the first side of the self-imaging semiconductorwaveguide; and a plurality of pump arrays arranged along themicrochannel cooler opposed from the first side of the self-imagingsemiconductor waveguide; wherein the quantum well gain material iscontinually photopumped by the pump arrays through a transparent heatsink disposed between the waveguide and the microchannel cooler, therebyavoiding electrical injection.
 5. The apparatus according to claim 4,wherein each of the pump arrays has a respective coupling optics.
 6. Theapparatus according to claim 4, wherein the microchannel cooler isformed of a copper alloy that is expansion matched to the semiconductorwaveguide.
 7. The apparatus according to claim 4, wherein themicrochannel cooler is formed of a copper alloy that includesmolybdenum, tungsten or diamond.
 8. The apparatus according to claim 4,wherein the self-imaging semiconductor waveguide is formed of a highthermal conductivity InP media material to permit high power handling.9. The apparatus according to claim 4, wherein the quantum wells usequaternary alloys to control band gap and lattice constant.
 10. Theapparatus according to claim 4, wherein the quantum wells have both bandgap and lattice constant mutually controlled using quaternary alloys.11. The apparatus according to claim 4, wherein a thickness of thequantum wells modifies a wavelength of the semiconductor waveguide.